GED Science
Genetics & Heredity
You get two copies of each gene, one from each parent, and some versions shout louder than others. A Punnett square is just a four-box multiplication table showing every combination a child could inherit. Fill the boxes and count.
The words first
- gene / allele
- an instruction / one version of it (B or b)
- dominant
- the loud version — shows up with just one copy, written CAPITAL (B)
- recessive
- the quiet version — needs TWO copies to show, written lowercase (b)
- genotype vs phenotype
- the letters (Bb) vs the visible trait (brown eyes)
- homozygous / heterozygous
- two same letters (BB, bb) / two different (Bb)
The method
READING LETTER PAIRS (B = brown, dominant; b = blue)
BB → brown (two loud copies)
Bb → brown (one loud copy is enough)
bb → blue (only shows because no B is present)
BUILDING A PUNNETT SQUARE for Bb × Bb
B b ← one parent's two letters
┌───────┬───────┐
B │ BB │ Bb │
├───────┼───────┤
b │ Bb │ bb │
└───────┴───────┘
↑ other parent's letters
count the boxes: 1 BB : 2 Bb : 1 bb
phenotypes: 3 brown : 1 blue → 25% chance of blue
Worked all the way through
The problem
Both parents have brown eyes with genotype Bb. What is the chance their child has blue eyes?
- Step 1 — Write each parent's letters on the square's edges.Top: B and b. Side: B and b.
- Step 2 — Fill each box with one letter from the top and one from the side.BB, Bb, Bb, bb.
- Step 3 — Find which boxes give blue eyes.Blue is recessive, so ONLY bb shows blue. That is 1 box.
- Step 4 — Count boxes into a probability.1 blue box out of 4 total = 1/4 = 25%.
On the board
parents: Bb × Bb
boxes: BB, Bb, Bb, bb
every combination
blue needs bb
recessive needs both
1 of 4 boxes
25% chance
Answer
25% (1 in 4)
A second one, in a different form
A bb (blue-eyed) parent and a BB (brown-eyed) parent have children. What eye colors are possible?
one parent gives only b, the other only B
every child: Bb
all four boxes identical
Bb shows brown
one B is enough
100% brown-eyed (all carriers)
Answer
All children have brown eyes — every one is Bb, carrying a hidden b
The usual mistake
Assuming two dominant-looking parents cannot have a recessive child. Bb × Bb gives 25%.
Check yourself
The recessive trait can only appear in the double-lowercase box. If a child SHOWS a recessive trait, you instantly know both parents carried at least one lowercase letter — the test asks this backwards version constantly.
Practice it
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